This invention relates to 3.alpha.,6.alpha.-dihydroxy-5.beta.-cholestan-24-one or its derivatives represented by the formula ##STR2## wherein R is hydrogen or a protective group for hydroxyl radical and to a process for preparing the same.
In recent years, extensive research has been carried on for developing compounds which are active metabolites of vitamine D. Especially, steroid compounds having hydroxyl group(s) attached to their side chain, particularly, cholesterol compounds having hydroxyl group(s) at 24 and/or 25 positions, have attracted researchers' attention as an intermediate for the production of active vitamin D.sub.3, for example, 1.alpha.,25-dihydroxycholecalciferol, 1.alpha.,24-dihydroxycholecalciferol, 1.alpha.,24,25-trihydroxycholecalcif erol, 25-hydroxycholecalciferol, 24,25-dihydroxycholecalciferol or the like.
Desmosterol is known as a useful intermediate for preparing these active vitamin D.sub.3 above. In fact, various types of active vitamin D.sub.3 are derived from desmosterol. Although desmosterol is very useful as an intermediate for preparing the active vitamin D.sub.3, it is difficult to commercially supply it as raw material because, for example, it is derived from fucosterol extracted from certain seaweeds, which is a hardly available natural substance.
The inventors of this invention carefully searched for ways to produce desmosterol or its derivatives and, after intensive research, found that hyodeoxycholic acid, which is easily available, can be used to prepare the compound represented by the formula (I) with several reaction steps. Further, they succeeded in easily preparing desmosterol using the compound (I) above to complete this invention.
According to this invention, the object compound (I) is prepared by reacting a compound represented by the formula ##STR3## wherein R' is a protecting group for hydroxyl radical and X is halogen, alkoxyl or aralkyloxyl, with an organometallic compound. The organometallic compounds which are useful in this invention include, for example, diisopropyl cadmium, diisopropyl zinc, isopropyl zinc halide and isopropyl magnesium halide.
The compound (II) may be easily prepared by introducing a protective group into hydroxyl groups of hyodeoxycholic acid by the conventional way and then reacting the acid with thionyl chloride, phosphorus oxychloride, phosphorus trichloride or phosphorus pentachloride under heating in the presence of or in the absence of a solvent, for example, benzene, chloroform, etc.
In the practice of the process of this invention, the compound (II) may be reacted with an organometallic compound in the presence of an inert solvent such as benzene, toluene or diethyl ether at a temperature ranging from 0.degree. to 50.degree. C., preferably, from 10.degree. to 30.degree. C. for 1-3 hours to give the object compound (I), the hydroxyl groups of which are protected. If necessary, the compound (I) with the protecting groups may be hydrolized in a conventional manner to remove the protecting group.
Incidentally, if the compound (I) with protecting groups is desired, said group may be conveniently selected from acyl, triarylmethyl, methoxymethyl, tetrahydropyranyl, alkysilyl or benzyl.
Some examples of process for preparation of desmosterol from the compounds of this invention are shown hereunder.
(a) 3.alpha.,6.alpha.-Dihydroxy-5.beta.-cholestan-24-one (I) is reduced for example by the action of an alkali metal hydride to form 3.alpha.,6.alpha.,24-trihydroxy-5.beta.-cholestane (III). The reduction may be carried out by adding to a reaction system an alkali metal borohydride, such as potassium borohydride or sodium borohydride, or lithium alminium hydride. The reaction is effected in a solvent such as an ether or an alcohol. Then, the compound (III) is reacted with a chlorinating agent such as phosphorus trichloride, phosphorus pentachloride or phosphorus oxychloride to form 3-chlorocholest-5,24-diene (IV). In this reaction, the hydroxyl groups of compound (III) are chlorinated and then the resulting 3,6,24-trichloro compound is easily dehydrochlorinated simultaneously to give the compound (IV). In the last reaction, any solvent inert to the reaction system can be used. However, an aromatic solvent such as benzene or toluene, or halogenated hydrocarbon such as chloroform, carbon tetrachloride or methylene chloride is usually used, and more preferably, a basic solvent or a base-containing solvent which is an acceptor for hydrogen chloride generated during the reaction is used. The reaction temperature may vary within the range from room temperature to the reflux temperature of the solvent used.
Then, the compound (IV) is converted to 3.beta.-acyloxycholest-5,24-diene (V). This substitution can be effected in a conventional manner, for example, by using potassium acetate in glacial acetic acid. It is confirmed that the compound (V) is the same compound as the acylated desmosterol.
(b) 3.alpha.,6.alpha.-Dihydroxy-5.beta.-cholestan-24-one (I) is mesylated in a conventional manner to form 3.alpha.,6.alpha.-dihydroxy-5.beta.-cholestan-24-one dimesylate (VI). Then the compound (VI) is reacted with an inorganic salt such as potassium chloride, sodium chloride or lithium chloride to give 3-chlorocholest-5-en-24-one (VII).
Although the reaction may be carried out in the absence of a solvent, the presence of a solvent is preferred to mildly accelerate the reaction. The kind of solvent suitable for this reaction is one that can easily dissolve an inorganic salt and may be selected from an amide such as dimethylformamide, hexamethylphosphoric triamide or hexamethylphosphoramide, an acetone or an alcohol. The reaction may be carried out at a temperature of from room temperature to an elevated temperature.
The compound (VII) which is prepared above is treated as in (a) above to introduce an acyloxyl group into 3.beta.-position thereby forming 3.beta.-acyloxycholest-5-en-24-one (VIII). The desmosterol derivative (V) as disclosed in (a) above may be easily derived from the compound (VIII) in a conventional manner, for example, as disclosed in Japanese Patent Disclosures Nos. 149662/1975 and 24264/1975.
(c) 3.alpha.,6.alpha.-Dihydroxy-5.beta.-cholestan-24-one diacetate (I) may be subjected to reduction reaction as in (a) above to give 3.alpha.,6.alpha.,24-trihydroxy-5.beta.-cholestane 3.alpha.,6.alpha.-diacetate (IX). The reduction reaction may be conveniently carried out without causing removal of an acyl group, if calcium borohydride is used. The compound (IX) is dehydrated with the aid of an acid catalyst to form 3.alpha.,6.alpha.-dihydroxy-5.beta.-cholest-24-ene diacetate (X). The presence of a tertiary amine such as pyridine or triethyl amine during the dehydration is preferred in order to increase the reaction rate and to improve the yield. Although any acid catalyst which may be used for such type of dehydration as that above may be utilized, the preferable acid catalysts include, for example, phosphorus oxychloride, thionyl chloride, methansulfonic acid, p-toluenesulfonic acid or its chloride, phosphoric anhydride and sulfuric acid. The compound (X) is hydrolized to form 3.alpha.,6.alpha.-dihydroxy-5.beta.-cholest-24-ene (XI). The compound (XI) is mesylated in a conventional manner to give 3.alpha.,6.alpha.-dihydroxy-5.beta.-cholest-24-ene dimesylate (XII). The compound (XII) is treated as in (b) above with the use of an inorganic salt to give 3-chlorocholest-5,24-diene (IV) which is treated as in (a) above to give 3.beta.-acyloxycholest-5,24-diene (V).